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Date: Wed, 4 Mar 2020 16:38:01 +0100 From: Daniel Borkmann <daniel@...earbox.net> To: Toke Høiland-Jørgensen <toke@...hat.com>, Andrii Nakryiko <andrii.nakryiko@...il.com> Cc: Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>, Alexei Starovoitov <ast@...com>, Kernel Team <kernel-team@...com>, Yonghong Song <yhs@...com> Subject: Re: [PATCH v3 bpf-next 1/3] bpf: switch BPF UAPI #define constants used from BPF program side to enums On 3/4/20 10:37 AM, Toke Høiland-Jørgensen wrote: > Andrii Nakryiko <andrii.nakryiko@...il.com> writes: >> On Tue, Mar 3, 2020 at 3:01 PM Daniel Borkmann <daniel@...earbox.net> wrote: >>> >>> On 3/3/20 1:32 AM, Andrii Nakryiko wrote: >>>> Switch BPF UAPI constants, previously defined as #define macro, to anonymous >>>> enum values. This preserves constants values and behavior in expressions, but >>>> has added advantaged of being captured as part of DWARF and, subsequently, BTF >>>> type info. Which, in turn, greatly improves usefulness of generated vmlinux.h >>>> for BPF applications, as it will not require BPF users to copy/paste various >>>> flags and constants, which are frequently used with BPF helpers. Only those >>>> constants that are used/useful from BPF program side are converted. >>>> >>>> Signed-off-by: Andrii Nakryiko <andriin@...com> >>> >>> Just thinking out loud, is there some way this could be resolved generically >>> either from compiler side or via additional tooling where this ends up as BTF >>> data and thus inside vmlinux.h as anon enum eventually? bpf.h is one single >>> header and worst case libbpf could also ship a copy of it (?), but what about >>> all the other things one would need to redefine e.g. for tracing? Small example >>> that comes to mind are all these TASK_* defines in sched.h etc, and there's >>> probably dozens of other similar stuff needed too depending on the particular >>> case; would be nice to have some generic catch-all, hmm. >> >> Enum convertion seems to be the simplest and cleanest way, >> unfortunately (as far as I know). DWARF has some extensions capturing >> #defines, but values are strings (and need to be parsed, which is pain >> already for "1 << 1ULL"), and it's some obscure extension, not a >> standard thing. I agree would be nice not to have and change all UAPI >> headers for this, but I'm not aware of the solution like that. > > Since this is a UAPI header, are we sure that no userspace programs are > using these defines in #ifdefs or something like that? Hm, yes, anyone doing #ifdefs on them would get build issues. Simple example: enum { FOO = 42, //#define FOO FOO }; #ifndef FOO # warning "bar" #endif int main(int argc, char **argv) { return FOO; } $ gcc -Wall -O2 foo.c foo.c:7:3: warning: #warning "bar" [-Wcpp] 7 | # warning "bar" | ^~~~~~~ Commenting #define FOO FOO back in fixes it as we discussed in v2: $ gcc -Wall -O2 foo.c $ There's also a flag_enum attribute, but with the experiments I tried yesterday night I couldn't get a warning to trigger for anonymous enums at least, so that part should be ok. I was about to push the series out, but agree that there may be a risk for #ifndefs in the BPF C code. If we want to be on safe side, #define FOO FOO would be needed. Thanks, Daniel
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